Chemical flocculation using inorganic coagulating chemicals has the advantage of providing a rapid sedimentation of particles and phosphorous containing compounds. The particle reducing effect is of great importance as 60 to 75% of COD of sewage water depends on particles. Sewage water contains organic compounds as well, of which a smaller part can be precipitated using inorganic salts. The use of these salts only can reduce more than 60 to 75% of COD of sewage water.
The use of a coagulating chemical is, however, not only restricted to the purification of sewage water but is also used in the purification of raw water for the production of drinking water. Further, coagulating chemicals are used in the manufacture of pulp in order to flocculate the cellulose fibre as well as in paper production for the flocculation of the cellulose fibre and filling agents present and at the hydrophobation of paper to attach resinous sizing agents onto the cellulose fibres. Further flocculation agents are Used to dewater organic matter, as well as they are used within the mining industry at the concentration of minerals.
It is previously known (FI pat. appl. 895163) solutions of polyaluminium silicate sulphate (PASS) to be used as flocculation agents at the purification of water, the product being produced by high shearing mixing of aluminium sulphate, sodium silicate, and sodium aluminate. Other fields of use are pulp and paper industry as well as the mining industry.
These PASS solutions are said to be efficient at low temperatures, less corrosive than iron sulphate, aluminium chloride and polyaluminium chloride, to provide a low residual aluminium content in the effluent, to require a low dosage, to provide formation of thick and rapidly settling flocs, to provide improved adsorption, to be effective over a broader pH range, to require a reduced need for alkali, and to be chlorine free, which is a positive matter within the paper industry.
These PASS solutions contains the active compound Al(OH).sub.x (SiO.sub.u).sub.y (SO.sub.4).sub.z (H.sub.2 O).sub.v +Na.sub.2 SO.sub.4, wherein x is 0.75 to 2.0, y is 0.005 to 0.1, and z is 0.3 to 1.12. The product is prepared by mixing different molar parts of aluminium sulphate, sodium silicate, and sodium aluminate and expose the mixture for a high shearing mixing at 10.degree. to 20.degree. C., and then allow the resulting mixture to ripe at 20.degree. to 90.degree. C. until it becomes a clear solution.
Compared with the production of aluminium sulphate which has the same properties and the same fields of use, even if less efficient, PASS is 1.5 to 1.6 times as expensive as aluminium sulphate.
It is thus desired with reference to the positive effects of PASS but with regard to the high costs to be able to produce a coagulation chemical which is as good as or better than PASS and which can be obtained at lower costs and in a more simple production way. The utility of a high shearing mixing means reduced capacity and high investment.